1. Field of the Invention
The present invention relates generally to an autofocus apparatus of a camera, and more particularly to an autofocus apparatus of a camera, which detects a direction and an amount of defocus of a photo-taking lens of the camera, and drives the lens on the basis of the direction and the amount of detected defocus.
2. Description of the Related Art
The range-finding principle of a TTL (through the taking lens) phase difference detection will be described below with reference to FIGS. 3-5. A pair of separator lenses 102, which are provided symmetrically with an optical axis 101 at spacing S with each other, are located at a distance f.sub.0 rearward from an equivalent surface of a film surface 100. A focus sensor 103, which includes a pair of photoelectric sensor arrays, is located at a distance f.sub.j rearward from the separator lenses 102. The reference numeral 104 represents a taking lens of a camera.
When compared with various focusing conditions, such as an in-focus condition (FIG. 3), a front focus condition (FIG. 4), and a rear focus condition (FIG. 5), the spacing of images 105 on the focus sensor 103 can be respectively characterized as front focus, in-focus, and rear focus, so that defocus amount is determined by measuring the spacing of images 105.
FIG. 6 illustrates the focus detecting principle in detail. The focus sensor 103 has a pair of sensors, unit a and unit b, each unit including m sensors of pitch p.
The output signals obtained from unit a and unit b from the output signal that is nearest to the optical axis 101 to the output signal that is farthest from optical axis 101 are defined as follows; EQU V.sub.a1, V.sub.a2, V.sub.a3, . . . V.sub.an, . . . V.sub.am EQU V.sub.b1, V.sub.b2, V.sub.b3, . . . V.sub.bn, . . . V.sub.bm
where m&gt;n.
Now, a shift operation is carried out by using n sensor signals out of m sensor signals of unit a and unit b, which is given by the following equation: EQU .DELTA.V.sub.j =.SIGMA..vertline.V.sub.ai+j -V.sub.bi+j.vertline. (1)
where j=1-n.
FIG. 7 illustrates the result of the shift operation given above. When the minimum value of .DELTA.V.sub.j is defined as .DELTA.V.sub.jo, the following equation is obtained by applying similarity of triangles as follows: EQU S/2/f.sub.x =Pj.sub.o /f.sub.j EQU f.sub.x =Sf.sub.j /(2Pj.sub.o) (2)
Defocus amount .DELTA.f.sub.ct, which is defined as the difference between a film plane and a focus plane of a taking lens, is given below: EQU .DELTA.f.sub.c =f.sub.o -f.sub.x EQU =f.sub.o -Sf.sub.j /(2Pj.sub.o) (3)
The required drive amount of the lens for making them in-focus is given as follows: EQU K=.DELTA.fc.beta..sup.2 (f.sub.TL) EQU =.beta..sup.2 (f.sub.TL){f.sub.o -Sf.sub.j /(2Pj.sub.o)} (4)
where the longitudinal magnification of the taking lens to the film surface is defined as .beta..sup.2 (f.sub.TL).
Moreover, the brightness of the object is given by measuring the amplitude of the output signals from the focus sensors.
FIG. 8(a) illustrates a lens drive apparatus in detail. The CPU 6 outputs to the IFIC (interface IC) 17 a signal indicative of the driving direction of the lens. The IFIC 17 then causes the motor 18 to apply a driving voltage. Thus, as the motor 18 begins to rotate, a screw 23 rotates via a shaft 26, so that the taking lens group 5, which is fixed to a lens barrel 22, moves along the optical axis 101 of the taking lens.
FIG. 8(b) illustrates a PI (photo-interrupter) 25 which is provided in the vicinity of a PI blade 24 provided on the shaft 26. The PI 25 corresponds to the pulse generator 27. Since the PI blade 24 has some periodic holes on it, the PI 25 outputs pulse signals in response to rotation of the PI blade 24. When the motor 18 rotates, the PI 25 outputs pulse signals. The CPU 6 counts the number of pulse signals from PI 25, so that the movement of the taking lens is determined. A predetermined number of pulse signals are counted in order to cancel backlash of the taking lens.
Japanese unexamined Patent Publication hei 4-306608 discloses a focus detecting apparatus which drives a photo-taking lens according to defocus measurements. The apparatus drives the lens according to a re-detection of a defocus occurring after correcting the backlash of the lens when the driving direction of the lens is altered.
Japanese unexamined Patent Publication sho 60-52812 discloses a focus detecting apparatus which drives a photo-taking lens according to defocus measurements. The apparatus drives the lens with adding backlash amount of the lens when the driving direction of the lens is altered.
In the focus detecting apparatus according to hei 4-306608, however, the driving direction of the lens is determined after a release button of the camera is actuated. If the driving direction of the lens is altered, then, at first, the lens is driven by as much as the backlash amount. Following this lens drive operation, a re-detection of the defocus amount and a driving of the lens are performed. Accordingly, the time lag between operation timing of the release button and in-focus timing tends to be relatively long.
In the focus detecting apparatus according to sho 60-52812, however, the backlash amount of the lens is added to the driving amount of the lens if the driving direction of the lens is altered. Accordingly, the accuracy in focusing tends to be reduced, and redetection of the defocus amount for assuring focus state of the lens is further required.